Mutants of the constitutive enzyme, lysyl-tRNA synthetase have been isolated from Escherichia coli K-12 which appear to be regulatory. The activity of the synthetase in some of these mutants is only 1 to 5 percent of wild-type. The level of the synthetase can be metabolite controlled, the most effective metabolites being leucine dipeptides and alanine in that order. The addition of leucine dipeptides to minimal medium can result in a 20 to 70 fold increase in the synthetase activity in various mutants. The increase in activity has been shown to be due to an increase in the synthesis of lysyl-tRNA synthetase rather than activation of inactive precursors. Recent experiments have shown that the addition of alanine to minimal medium can influence the properties of wild-type lysyl-tRNA synthetase and at the same time stimulate its formation 2 to 3 fold. Thus there appears to be some functional relationship between the apparent modification of lysyl-tRNA synthetase by alanine, and the ability of this metabolite to stimulate the synthesis of the enzyme. It is proposed to investigate the mechanism and nature of the modification of lysyl-tRNA-synthetase by alanine, and also to determine if leucine dipeptides act in the same fashion. It is of further interest to determine if the metabolites stimulate the synthesis of the synthetase transcriptionally. It is proposed to investigate this by using a combination of biochemical and genetic techniques with the ultimate purpose of demonstrating that the transcription of lysyl-tRNA synthetase messenger RNA in a cell-free system requires alanine or leucine peptides at least in part if not completely. At the present time metabolites are not considered to be of significance in the control of constitutive enzyme synthesis. If these studies reach fruition, they will provide a model for the control of at least one constitutive enzyme in which metabolites do play a significant role.